Graphene is a single-layer sp2-hybridized 2D network of carbon atoms that serves as the basis of many allotropes of carbon. It can be stacked to form 3D graphite, rolled to form 1D carbon nanotubes, and fused to form 0D fullerenes. Owing to its strongly delocalized electron configuration, graphene exhibits exceptional charge carrier mobility, thermal conductivity, mechanical strength, and chemical stability. However, like other nanomaterials, the properties of graphene are dependent on its dimensions, physical environment, and integration schemes. Graphene and graphene-based materials have tailorable properties that can be exploited in a broad range of devices, including transistors, capacitors, electron field emitters, transparent conductors, sensors, catalysts, and drug delivery agents. Other 2D crystals, most notably boron nitride and molybdenum disulfide, have also been isolated.
Two-dimensional (2D) sp2-bonded carbon exists in the form of graphene, buckyballs and carbon nanotubes (CNTs). Graphene is “flat” or 2D, fullerenes (“Buckyballs”) are spherical or OD, and CNTs are tubes in 1D. Forming these materials in a singular, regular, repeatable structure has not previously been achieved. Superstructures of these materials may provide very strong, light, highly thermally and electrically conductive structures. Attempts have been made to fabricate sp2-bonded sponges, however these structures are irregular with properties that vary with position.
The isolation of graphene via the mechanical exfoliation of graphite has sparked strong interest in two-dimensional (2D) layered materials. The properties of graphene include exceptional strength, and high electrical and thermal conductivity, while being lightweight, flexible and transparent. This opens the possibility of a wide range of potential applications, including high-speed transistors and sensors, barrier materials, solar cells, batteries, and composites.
Other classes of 2D materials of interest include transition metal dichalcogenide (TMDC) materials, hexagonal boron nitride (h-BN), as well as those based on Group 14 elements, such as silicene and germanene. The properties of these materials can range from semi-metallic, for example, NiTe2 and VSe2, to semiconducting, for example, WSe2 and MoS2, to insulating, for example, hexagonal boron nitride (h-BN).
Growth of regular 3D superstructures using sp2-bonded carbon may address the shortcomings of the flexible sp2 carbons for 3D applications given that hexagonally arranged carbon is strong, chemically inert, electrically and thermally conductive, and optically transparent. Such 3D superstructures may find used in a number of applications from packaging, thin optically transparent electrically conductive strong thin films, and many more.
When a carbon atom is attached to three groups (or, as in the case of graphene, three other carbon atoms) via three a bonds, it requires three orbitals in the hybrid set. This requires sp2 hybridization.
An sp2-hybridized bond has 33% s and 67% p character. The three sp2 hybrid orbitals point towards the corners of a triangle at 120° to each other. Each sp2 hybrid is involved in a σ bond. The remaining p orbital forms the n bond. A carbon double bond may be viewed as a σ+π bond.
A three-dimensional integrated circuit (3D IC) is an integrated circuit manufactured by stacking silicon wafers or dies and interconnecting them vertically using, for instance, through-silicon vias (TSVs) or Cu—Cu connections, so that they behave as a single device to achieve performance improvements at reduced power and smaller footprint than conventional two-dimensional processes. A multi-layer or super lattice structure such as is disclosed herein may be used to create a switchable array in a 3D IC.
The junction gate field-effect transistor (JFET) is the simplest type of field-effect transistor. They are three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers, or voltage-controlled resistors.
Unlike bipolar transistors, JFETs are exclusively voltage-controlled in that they do not need a biasing current. Electric charge flows through a semiconducting channel between source and drain terminals and by applying a reverse bias voltage to a gate terminal, the channel is “pinched”, so that the electric current is impeded or switched off completely. A JFET is usually on when there is no potential difference between its gate and source terminals. If a potential difference of the proper polarity is applied between its gate and source terminals, the JFET will be more resistive to current flow, which means less current would flow in the channel between the source and drain terminals. Thus, JFETs are sometimes referred to as depletion-mode devices.
JFETs can have an n-type or p-type channel. For n-type devices, if the voltage applied to the gate is less than that applied to the source, the current will be reduced (similarly for p-type devices, if the voltage applied to the gate is greater than that applied to the source). A JFET has a large input impedance (sometimes on the order of 1010 ohms), which means that it has a negligible effect on external components or circuits connected to its gate.
The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers or holes (p-type), or of negative carriers or electrons (n-type). Ohmic contacts are formed at the source (S) and the drain (D). A p-n-junction is formed on one or both sides of the channel, or surrounding it, using a region with doping opposite to that of the channel, and biased using an ohmic gate contact (G).